Cerium oxide particles and method for production thereof

ABSTRACT

The present invention relates to cerium oxide particles that have excellent heat resistance especially useful for catalysts, functional ceramics, solid electrolyte for fuel cells, polishing, ultraviolet absorbers and the like, and particularly suitable for use as a catalyst or co-catalyst material, for instance in catalysis for purifying vehicle exhaust gas. The present invention also relates to a method for preparing such cerium oxide particles, and a catalyst, such as for purifying exhaust gas, utilizing these cerium oxide particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national phase entry under 35 U.S.C. §371 of International Application No. PCT/EP2015/076272, filed on 10 Nov.2015, which claims priority to European application No. 14290344.2,filed on 12 Nov. 2014. The entire content of each of these applicationsis explicitly incorporated herein by reference.

The present invention relates to cerium oxide particles that haveexcellent heat resistance especially useful for catalysts, functionalceramics, solid electrolyte for fuel cells, polishing, ultravioletabsorbers and the like, and particularly suitable for use as a catalystor co-catalyst material, for instance in catalysis for purifying vehicleexhaust gas. The present invention also relates to a method forpreparing such cerium oxide particles, and a catalyst, such as forpurifying exhaust gas, utilizing these cerium oxide particles.

PRIOR ART

The following discussion of the prior art is provided to place theinvention in an appropriate technical context and enable the advantagesof it to be more fully understood. It should be appreciated, however,that any discussion of the prior art throughout the specification shouldnot be considered as an express or implied admission that such prior artis widely known or forms part of common general knowledge in the field.

Catalysts for purifying vehicle exhaust gas are composed of a catalyticmetal such as platinum, palladium, or rhodium, and a co-catalyst forenhancing the catalytic action of such metal, both supported on acatalyst support made of, for example, alumina or cordierite. As such aco-catalyst material are used cerium oxide-containing materials, whichhave the properties of absorbing oxygen under the oxidizing atmosphereand desorbing oxygen under the reducing atmosphere, originated in ceriumoxide, i.e., oxygen absorbing and desorbing capability. With this oxygenabsorbing and desorbing capability, the cerium oxide-containingmaterials purify noxious components in exhaust gas such as hydrocarbons,carbon monoxide, and nitrogen oxides at excellent efficiency. As such,large quantities of the cerium oxide-containing materials are used as aco-catalyst.

It is most critical for activating the function of such ceriumoxide-containing co-catalyst material to keep the co-catalyst at a hightemperature. Low temperature of the exhaust gas, for example at enginestart-up, will result in low purifying efficiency. Vehicle manufacturersare presently trying to solve this problem by placing the catalystsystem close to the engine for introducing hot exhaust gas right afterits emission from the engine into the catalyst system. There is also ademand for co-catalyst materials that are activated at lowertemperatures.

In general, efficiency of exhaust gas treatment with a catalyst isproportional to the contact area between the active phase of thecatalyst and the exhaust gas, and to the oxygen absorbing and desorbingcapability of the co-catalyst material, such as cerium oxide. Thus theco-catalyst material is required to have a sufficiently large specificsurface area and a sufficiently high oxygen absorbing and desorbingcapability, as well as high activity at lower temperatures.

For solving these problems, U.S. Pat. No. 7,361,322 B2 proposes a methodfor obtaining a cerium oxide having good heat resistance with a specificsurface area higher than 30.0 m²/g after calcination at 900° C. for 5hours, especially around 40-50 m²/g, comprising the steps of:

(a) providing a cerium solution wherein not less than 90 mol % of thecerium are tetravalent cerium cations, said cerium solution having acerium concentration of 10 to 60 g/L in terms of cerium oxide;

(b) holding said cerium solution prepared in step (a) at 60 to 220° C.under heating;

(c) cooling said heated cerium solution;

(d) adding a precipitant to said cooled cerium solution to obtain aprecipitate; and

(e) calcining said precipitate.

However it appears that heat resistance of specific surface area ofthese cerium oxides obtained by this process are still not sufficient.

Also, heat resistance of total pore volume has also come to be requiredin addition to heat resistance of specific surface area of catalystsupports. High heat resistance of total pore volume usually means thatdecrease ratio of pore volume in comparison of two different ageingconditions of catalyst supports, such as fresh and 800° C., is small. Inthe case of loading an active species in the form of a precious metal,such as active metal, onto a catalyst support, the precious metal isloaded with good dispersibility into pores. Thus, a cerium oxide havinga large pore volume even at high temperatures is desirable.

There is still a need to provide cerium oxides having higher heatresistance and oxygen absorbing and desorbing capability useful as acatalyst or a co-catalyst material suitable for a catalyst, such as forpurifying exhaust gas.

INVENTION

It is therefore an object of the present invention to provide ceriumoxide (cerium(IV) oxide) that has excellent heat resistance andabsorbing and desorbing capability, useful for catalysts, functionalceramics, solid electrolyte for fuel cells, polishing, ultravioletabsorbers and the like, and particularly suitable for use as a catalystor co-catalyst material, particularly in catalysis for purifying vehicleexhaust gas. Cerium oxides particles of the present invention alsoprovide high heat resistance of total pore volume and specific surfacearea. Cerium oxides particles of the invention are in particular capableof maintaining a large specific surface area even in use in a hightemperature environment. These cerium oxide particles are also capableof exhibiting high oxygen absorbing and desorbing capability in a lowertemperature range. Invention also concerns a method for preparing thesecerium oxide particles, and a catalyst for purifying exhaust gasutilizing said cerium oxide particles.

Cerium oxide particles of the invention also provide a high NOx captureperformance, permitting then reduction of NOx emission from automobilesin order to follow stringent pollutants regulations. These cerium oxideparticles are then also useful for NOx trap (LNT) catalysts.

The present invention then concerns cerium oxide particles having thefollowing properties:

-   -   a specific surface area (SBET) comprised between 80 and 120 m²/g        after calcination at 800° C. for 2 hours, under air;    -   a specific surface area (SBET) comprised between 55 and 80 m²/g        after calcination at 900° C. for 5 hours, under air;    -   a total pore volume comprised between 0.9 and 1.6 ml/g after        calcination at 800° C. for 2 hours, under air; and    -   a total pore volume comprised between 0.85 and 1.5 ml/g after        calcination at 900° C. for 5 hours, under air.

The present invention also concerns a method for preparing cerium oxideparticles, comprising at least the steps of:

(a) providing a cerium salt solution comprising anions and cations,wherein between 90 and 100 mol % of the cerium cations are tetravalentcerium cations;

(b) heating said cerium salt solution at a temperature comprised between60 and 220° C. in order to obtain a suspension comprising a liquidmedium and a precipitate;

(c) decreasing the concentration of anions from the cerium salt presentin the liquid medium between 10 and 90 mol %, in comparison with saidanions comprised in the liquid medium in step (b);

(d) heating the suspension obtained in step (c) at a temperaturecomprised between 100 and 300° C.;

(e) optionally cooling the suspension obtained in the step (d);

(f) bringing said suspension into contact with a basic compound;

(g) optionally separating off the precipitate from the liquid medium;

(h) adding an organic texturing agent to the suspension obtained in step

(f) or the precipitate obtained in step (g);

(i) optionally separating off the precipitate from the liquid medium;and

(j) calcining the precipitate obtained at the end of step (h) orobtained at step (i) to obtain cerium oxide particles; said process ofthe invention comprising at least said step (g) and/or at said step (i).

The invention also concerns cerium oxide particles susceptible to beobtained by this process.

Other characteristics, details and advantages of the invention willemerge even more fully upon reading the description which follows.

DEFINITIONS

Throughout the description, including the claims, the term “comprisingone” should be understood as being synonymous with the term “comprisingat least one”, unless otherwise specified, and “between” should beunderstood as being inclusive of the limits.

It is specified that, in the continuation of the description, unlessotherwise indicated, the values at the limits are included in the rangesof values which are given.

The contents are given as oxides, unless otherwise indicated. The ceriumoxide is in the form of cerium oxide (CeO₂).

In the continuation of the description, the term “specific surface area”is understood to mean the BET specific surface area determined bynitrogen adsorption in accordance with standard ASTM D 3663-78 laid downfrom the Brunauer-Emmett-Teller method described in the periodical “TheJournal of the American Chemical Society, 60, 309 (1938)”.

As used herein, the term “alkyl” groups is intended to mean: a saturatedaliphatic hydrocarbon-based group containing between 1 and 22 carbonatoms, advantageously between 1 and 10 carbon atoms, of formulaC_(n)H_(2n+1), obtained by removing a hydrogen from an alkane. The alkylgroup may be linear or branched. By way of example, the alkyl groupsinclude saturated hydrocarbons having one or more carbon atoms,including straight-chain alkyl groups, such as methyl, ethyl, propyl,butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, cyclic alkyl groups(or “cycloalkyl” or “alicyclic” or “carbocyclic” groups), such ascyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl,branched-chain alkyl groups, such as isopropyl, tert-butyl, sec-butyl,and isobutyl, and alkyl-substituted alkyl groups, such asalkyl-substituted cycloalkyl groups and cycloalkyl-substituted alkylgroups. In complex structures, the chains may be branched or bridged.

DETAILS OF THE INVENTION

According to the present method, first a cerium salt solution,comprising at least anions and cations, such as cerium cations, whereinbetween 90 and 100 mol % of the cerium cations are tetravalent isprovided in step (a). In step (a), the cerium salt solution, maypreferably be a cerium nitrate solution, a cerium ammonium nitratesolution, a cerium sulfate solution and/or a cerium ammonium sulfatesolution. Cerium salts are ionic compounds usually resulting from theneutralization reaction of an acid and a base or dissolution of a ceriumcompound, such as cerium hydroxide, with an acid. They are composed ofcerium cations and anions so that the product is electrically neutral.Said cerium salt solution is preferably an aqueous cerium salt solutionin which the liquid medium is water.

Cerium salt solution of the present invention may have a cerium cationsconcentration comprised between 5 and 150 g/L expressed in terms ofcerium oxide. For instance, a concentration of 225 g/L of cerium nitratecorresponds to 100 g/L of CeO₂; a concentration of 318 g/L of ceriumammonium nitrate corresponds to 100 g/L of CeO₂; a concentration of 193g/L of cerium sulfate corresponds to 100 g/L of CeO₂; a concentration of270 g/L of cerium ammonium sulfate corresponds to 100 g/L of CeO₂.

The cerium salt concentration of the cerium solution may be adjustedbetween 10 and 120 g/L, more preferably between 15 and 100 g/L, in termsof cerium oxide, usually with water, preferably with deionized water.

Cerium salt solution in step (a) may have an initial acid concentrationusually between 0.01 and 1 N. Acids, such as HNO₃ or H₂SO₄, may comefrom the cerium salt raw material solution or added as a stabilizer ofthe solution.

According to the present method, next the cerium salt solution preparedin step (a) is held between 60 and 220° C. under heating to causereaction of the cerium solution in step (b), in order to obtain asuspension comprising a liquid medium and a precipitate especiallycomprising cerium hydroxide.

Any reaction vessel may be used in step (b) without critical limitation,and either a sealed vessel or an open vessel may be used. Specifically,an autoclave reactor may preferably be used.

In step (b), the temperature is comprised between 60 and 220° C.,preferably between 80 and 180° C., more preferably between 90 and 160°C. Duration of heat treatment is usually between 10 minutes and 48hours, preferably between 30 minutes and 36 hours, more preferablybetween 1 hour and 24 hours. If the cerium solution is not sufficientlyheld under heating, the crystallinity of the precipitate may not beimproved, resulting in insufficient heat resistance of the objectivecerium oxide.

In step (c), the concentration of the anions from the cerium saltpresent in the liquid medium is decreased between 10 and 90 mol %,preferably between 15 and 85 mol %, in comparison with anions comprisedin the liquid medium in step (b). If the same amount of anions ispresent in step (a) and (b), the decrease of anions in the liquid mediumin step (c) may be calculated in comparison with anions comprised in theliquid medium in step (a) or (b).

Anions from the cerium salts may be for instance nitrate from ceriumnitrate or sulfate from cerium sulfate.

This decrease of anions concentration present in the liquid medium maybe obtained by at least one of the following methods:

-   -   addition of water, preferably deionized water, to the suspension        obtained in step (b); and/or    -   removing at least a part of the liquid medium from the        suspension obtained in step (b) and then adding water,        preferably deionized water, to the medium. Said medium is        defined according to partial removal or complete removal of        liquid medium as previously expressed; i.e. medium may be a        precipitate in case of complete removal of liquid medium in step        or rather a mixture of precipitate and liquid medium in case of        partial removal of liquid medium.

Separation of the liquid medium from the precipitate may be carried out,for example, by Nutsche filter method, centrifuging, filter pressing, ordecantation.

According to the invention, partial removal or complete removal ofliquid medium is understood to mean that the liquid medium is partially,or completely removed from the precipitate. For example between 10 and100% by weight, preferably between 10 and 90% by weight, more preferablybetween 15 and 95% by weight, especially between 20 and 90% by weight,of the liquid medium present in step (b) may be removed in step (c).

Decrease of concentration of anions present in the liquid medium in step(c) in comparison with anions comprised in the liquid medium in step(b), may be calculated as follows in case of cerium nitrate:

Materials at the start of step (a) are Ce(IV)(NO₃)₄ and Ce(III)(NO₃)₃and optionally HNO₃.

1. Calculation of total number of NO₃ ⁻ ions (mol)NO₃ ⁻ (mol)=A/172.12*[B/100*4+(100−B)/100*3]+C=D

wherein:

-   -   A is quantity of cerium cations in terms of CeO₂ (gram), in step        (a)    -   B is percentage of tetravalent cerium cations per total cerium        cations, at the start of step (b)    -   C is quantity of HNO₃ (mol) if any, in step (a)

2. Calculation of NO₃ ⁻ concentration in step (b)[NO₃ ⁻] (mol/l)=D/E

wherein E is volume (liter) of reaction medium in step (b). B may bemeasured directory such as using measuring cylinder, or gauge.

3. Calculation of NO₃ ⁻ concentration in step (c)[NO₃ ⁻] (mol/l)=F/G

-   -   F is quantity of NO₃ ⁻ ions (mol). F=D if the liquid medium is        not removed. F=D*removal ratio of liquid medium if the liquid        medium is removed.    -   G is volume (liter) after adding of water.

4. Decrease ratio of NO₃ ⁻ concentration

-   -   decrease ratio of [NO₃ ⁻] (%)=[NO₃ ⁻] in step (c)/[NO₃ ⁻] in        step (b)*100=(F/G)/(D/E)*100

It is also possible to proceed with a direct measurement of NO₃ ⁻concentration of step (b) and (c). NO₃ ⁻ concentration can be analyzedby ion chromatography or adsorptiometer, both apparatus being commonlyused to analyze NO₃ ⁻ concentration in the liquid medium. A part of theliquid medium is put in the analyzer to automatically measure the NO₃ ⁻concentration. It is then possible to compare the both concentrations tocalculate the decrease ratio of NO₃ ⁻ concentration.

In step (d), the suspension is heated at a temperature comprised between100 and 300° C., preferably comprised between 110 and 150° C. Anyreaction vessel may be used without critical limitation, and either asealed vessel or an open vessel may be used. Specifically, an autoclavereactor may preferably be used. The duration of heat treatment isusually between 10 minutes and 48 hours, preferably between 30 minutesand 36 hours.

Following step (d), the heated suspension may be cooled in an optionalstep (e). The suspension may usually be cooled under stiffing. Means forcooling are not critical, and it may be cooling in an atmosphere orforced cooling with cooling tube. The temperature of the suspensionafter cooling may be comprised between 20 and 90° C.

According to step (f), a basic compound is then added to the suspension,or the suspension having been cooled.

This basic compound may be for example sodium hydroxide, potassiumhydroxide, an aqueous ammonia solution, ammonia gas, or mixturesthereof, with an aqueous ammonia solution being preferred. The basiccompound may be added by first preparing an aqueous solution of thebasic compound at a suitable concentration and adding the solution tothe cooled suspension prepared in step (e) under stirring, or whenammonia gas is used, by blowing the ammonia gas into the reaction vesselunder stirring. The amount of the basic compound may easily be decidedby tracing the pH change of the solution. Usually, a sufficient amountis such that the pH of the solution is not lower than 7, and a preferredamount is such that the pH is between 7 and 9.

Basic compounds are especially useful to precipitate Ce³⁺ ions which aredispersed in the suspension at the end of step (d) or (e) to form thenCe(OH)₃ precipitates.

In step (g), separation of the liquid medium from the precipitate, maybe carried out, for example, by Nutsche filter method, centrifuging,filter pressing, or decantation. The precipitate may optionally bewashed with water, preferably with water at basic pH, for exampleaqueous ammonia solution. Further, the precipitate may optionally bedried.

The suspension obtained in step (f) or the precipitate obtained in step(g) may be subjected to a step of heat treatment at a temperaturecomprised between 90 and 220° C., preferably between 100 and 180° C.,more preferably between 110 and 160° C. The duration of the heattreatment is usually between 10 minutes and 48 hours, preferably between30 minutes and 36 hours, more preferably between 1 and 24 hours.

It is also possible to add at any point between after step (c) andbefore step (h) of the process a rare earth element compound, forexample a rare earth element in the form of nitrate, chloride, oxide,hydroxide, carbonate, halide, oxyhalide, oxynitrate, and/or sulfate.Rare earth element (REE) or rare earth metal is one of a set ofseventeen chemical elements in the periodic table, meaning the fifteenlanthanides plus scandium and yttrium. Preferably, the rare earthelement oxide are chosen in the group consisting of: lanthanium oxide(La₂O₃), praseodymium oxide (Pr₆O₁₁), neodymium oxide (Nd₂O₃) andyttrium oxide (Y₂O₃).

Cerium oxide particles obtained by the process of the invention may thencomprise at least one rare earth element oxide, other than cerium oxide,for instance in a proportion comprised between 1 and 40% by weight ofoxide, preferably in a proportion comprised between 1 and 20% by weightof oxide. Oxide refers there to final mixed oxide defined as integrationof cerium oxide and rare earth element oxide.

In step (h), an organic texturing agent is added to the suspensionobtained in the preceding step (f) or the precipitate obtained in step(g) once separated from the liquid medium.

An organic texturing agent usually refers to an organic compound, suchas a surfactant, able to control or modify the mesoporous structure ofthe cerium oxide. “Mesoporous structure” basically describes a structurewhich specifically comprises pores with an average diameter comprisedbetween 2 and 50 nm, described by the term “mesopores”. Typically, thesestructures are amorphous or crystalline compounds in which the pores aregenerally distributed in random fashion, with a very wide pore-sizedistribution.

The organic texturing agent may be added directly or indirectly. It canbe added directly to the suspension or precipitate resulting from thepreceding step. It can also be first added in a composition, forinstance comprising a solvent of the organic texturing agent, and saidcomposition being then added to the suspension or precipitate aspreviously obtained.

The amount of organic texturing agent used, expressed as percentage byweight of additive relative to the weight of the cerium in terms ofCeO₂, is generally between 5 and 100% and more particularly between 15and 60%.

The organic texturing agent may be adsorbed on the surface of secondaryparticles and primary particles of the precipitates. For instance, theorganic texturing agent adsorbed on the primary particles will lead toincrease the size of mesopores and pore volume of the precipitate.

Organic texturing agents are preferably chosen in the group consistingof: anionic surfactants, nonionic surfactants, polyethylene glycols,carboxylic acids and their salts, and surfactants of thecarboxymethylated fatty alcohol ethoxylate type. With regard to thisadditive, reference may be made to the teaching of applicationWO-98/45212 and the surfactants described in this document may be used.

As surfactants of anionic type, mention may be made ofethoxycarboxylates, ethoxylated fatty acids, sarcosinates, phosphateesters, sulfates such as alcohol sulfates, alcohol ether sulfates andsulfated alkanolamide ethoxylates, and sulfonates such assulfosuccinates, and alkylbenzene or alkylnapthalene sulfonates.

As nonionic surfactants, mention may be made of acetylenic surfactants,alcohol ethoxylates, alkanolamides, amine oxides, ethoxylatedalkanolamides, long-chain ethoxylated amines, copolymers of ethyleneoxide/propylene oxide, sorbitan derivatives, ethylene glycol, propyleneglycol, glycerol, polyglyceryl esters and ethoxylated derivativesthereof, alkylamines, alkylimidazolines, ethoxylated oils andalkylphenol ethoxylates. Mention may in particular be made of theproducts sold under the brands Igepal®, Dowanol®, Rhodamox® andAlkamide®.

With regard to the carboxylic acids, it is in particular possible to usealiphatic monocarboxylic or dicarboxylic acids and, among these, moreparticularly saturated acids. Fatty acids and more particularlysaturated fatty acids may also be used. Mention may thus in particularbe made of formic acid, acetic acid, propionic acid, butyric acid,isobutyric acid, valeric acid, caproic acid, caprylic acid, capric acid,lauric acid, myristic acid and palmitic acid. As dicarboxylic acids,mention may be made of oxalic acid, malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid andsebacic acid.

Salts of the carboxylic acids may also be used, in particular theammonium.

By way of example, mention may be made more particularly of lauric acidand ammonium laurate.

Finally, it is possible to use a surfactant which is selected from thoseof the carboxymethylated fatty alcohol ethoxylate type.

The expression “product of the carboxymethylated fatty alcoholethoxylate type” is intended to mean products consisting of ethoxylatedor propoxylated fatty alcohols comprising a CH₂—COOH group at the end ofthe chain.

These products may correspond to the formula:R₁—O—(CR₂R₃—CR₄R₅—O)_(n)—CH₂—COOH

in which R₁ denotes a saturated or unsaturated carbon-based chain ofwhich the length is generally at most 22 carbon atoms, preferably atleast 12 carbon atoms; R₂, R₃, R₄ and R₅ may be identical and mayrepresent hydrogen or else R₂ may represent an alkyl group such as a CH₃group and R₃, R₄ and R₅ represent hydrogen; n is a non-zero integer thatmay be up to 50 and more particularly between 5 and 15, these valuesbeing included. It will be noted that a surfactant may consist of amixture of products of the formula above for which R₁ may be saturatedor unsaturated, respectively, or alternatively products comprising both—CH₂—CH₂—O— and —C(CH₃)—CH₂—O— groups.

Optionally in step (i) the precipitate is separated off from the liquidmedium, for example, by Nutsche filter method, centrifuging, or filterpressing. The precipitate may optionally be washed with an aqueoussolution, preferably with water at basic pH, for example aqueous ammoniasolution. Further, the precipitate may optionally be dried to a suitableextent for improving the efficiency in the following step.

In step (j), the precipitate obtained in the preceding step is calcinedto obtain the cerium oxide particles which are the object of theinvention.

Process of the invention then comprises either a step (g) of separationof the liquid medium from the precipitate, either a step (i) separatingoff the precipitate from the liquid medium, or both step (g) and step(i); in order to proceed with calcination of the precipitate obtained atthe end of step (h) or obtained at step (i) to obtain cerium oxideparticles.

In step (j), the calcination temperature may suitably be selected fromthe range of usually between 250 and 900° C. The selection of thetemperature may be made as desired, depending on the required values ofthe specific surface area and bulk density. From a practical point ofview to prepare a catalyst or a co-catalyst material wherein thespecific surface area is important, the calcination temperature in step(j) may preferably be between 250 and 800° C., more preferably between250 and 700° C., most preferably between 280 and 450° C. The duration ofcalcination may suitably be determined depending on the temperature, andmay preferably be between 1 and 20 hours.

After step (j), the cerium oxide particles obtained may usually bepulverized. The pulverization may sufficiently be performed in anordinary pulverizer, such as a hammer mill, to obtain a powder of adesired particle size. The cerium oxide obtained by the present methodmay be given a desired particle size through the above mentionedpulverization. For use as a co-catalyst in a catalyst for purifyingexhaust gas, for example, a preferred average particle size of thecerium oxide is between 0.5 and 50 μm.

Cerium oxide particles of the present invention have the followingproperties:

-   -   a specific surface area (SBET) comprised between 80 and 120 m²/g        after calcination at 800° C. for 2 hours, under air; preferably        comprised between 90 and 120 m²/g.    -   a specific surface area (SBET) comprised between 55 and 80 m²/g        after calcination at 900° C. for 5 hours, under air; preferably        comprised between 60 and 80 m²/g.    -   a total pore volume comprised between 0.9 and 1.6 ml/g after        calcination at 800° C. for 2 hours, under air; preferably        comprised between 1.1 and 1.6 ml/g.    -   a total pore volume comprised between 0.85 and 1.5 ml/g after        calcination at 900° C. for 5 hours, under air; preferably        comprised between 1.0 and 1.5 ml/g.

The total pore volume may be measured by ordinary mercury porosimeter.

Cerium oxide particles may have a S1/S2 ratio comprised between 0.5 and0.7 taken after calcination at 800° C. for 2 hours. Cerium oxideparticles may have a S1/S2 ratio comprised between 0.3 and 0.5 takenafter calcination at 900° C. for 5 hours.

Said S1/S2 ratio is a ratio of the area (S1) defined by a baseline and aTPR curve in a temperature range of 200 to 600° C. to the area (S2)defined by said baseline and said TPR curve in a temperature range of600 to 1000° C. A higher S1/S2 ratio of a cerium oxide is expected toresult in a higher oxygen absorbing and desorbing capability and higheractivity to purify exhaust gas at a lower temperature. As used herein,the “baseline” means a line segment drawn from the point on the TPRcurve corresponding to 200° C. in a parallel to the axis representingtemperature, up to 1000° C.

The cerium oxide of the present invention may preferably be prepared bythe production method according to the present invention to be discussedbelow, with good reproducibility and in an economical manner.

Cerium oxide particles may also comprise at least one rare earth elementoxide, other than cerium oxide, for instance in a proportion comprisedbetween 1 and 40% by weight of oxide, preferably in a proportioncomprised between 1 and 20% by weight of oxide. Oxide refers there tofinal mixed oxide defined as integration of cerium oxide and rare earthelement oxide.

Cerium oxide particles as described above or as obtained by means of thepreparation process previously described may be in the form of powders,but they can optionally be formed so as to be in the form of granules,pellets, foams, beads, cylinders or honeycombs of variable dimensions.

The present invention also concerns a catalyst comprising at leastcerium oxide particles as previously defined, such as a catalyst forpurifying exhaust gas.

Cerium oxide particles of the invention may be applied as such or withina composition to any support commonly used in the field of catalysis,that is to say in particular thermally inert supports. This support canbe chosen from alumina, titanium oxide, cerium oxide, zirconium oxide,silica, spinels, zeolites, silicates, crystalline silicoaluminumphosphates or crystalline aluminum phosphates.

The catalyst for purifying exhaust gas according to the presentinvention may be of any type, as long as it has a co-catalyst containingthe cerium oxide of the present invention. The catalyst may be produced,for example, by a commonly known method and with commonly known othermaterials.

The invention also concerns a composition, preferably a liquidcomposition, comprising at least cerium oxide particles as previouslyobtained and defined. More preferably said composition is a suspensioncomprising at least a liquid medium and cerium oxide particles aspreviously obtained and defined.

According to an embodiment of the invention, the invention also relatesto the use of cerium oxide particles as defined and/or as obtained inthe above identified process for the polishing application. Forinstance, a composition, such as a suspension, for polishing comprisingat least the cerium oxide particles of the invention may be obtained.This composition can be used for polishing glass, for example in thecrystal-making or mirror industry, flat glass, television screens orspectacles, or else for polishing ceramics or other materials ofvitreous type. This composition can also be used most particularly forCMP-type polishing in the electronics industry and therefore forpolishing metal substrates which go to make up microprocessors, but alsofor polishing insulating layers or Interlayer Dielectric (ILD) layers ofthese same microprocessors, the suspension of the invention beingparticularly suitable for the polishing of said layers. Chemicalmechanical planarization (CMP) is a key process enabling Shallow TrenchIsolation (STI), which is used in current integrated circuitmanufacturing processes to achieve device isolation. These layers aregenerally made of silica, such as doped silica or porous silica. Thissuspension may also be used for metal CMP for wiring and barrier inintegrated circuit, polishing a photomask substrate, especially made ofa synthetic quartz glass.

In general, such compositions comprise, in addition to the compound withabrasive property, such as the oxide particles, additives such as adispersing agent and/or an oxidant.

The present invention also concerns a method of removing a portion of asubstrate, for instance in a CMP operation, comprising:

-   -   providing at least a composition, for instance a suspension,        comprising cerium oxide particles of the invention,    -   contacting at least the composition and the substrate to be        polished, and    -   performing the polishing on the substrate.

The following examples are included to illustrate embodiments of theinvention. Needless to say, the invention is not limited to describedexamples.

EXPERIMENTAL PART Example 1

50 g of a ceric nitrate solution in terms of CeO₂ containing not lessthan 90 mol % tetravalent cerium cations was measured out, and adjustedto a total amount of 1 L with deionized water. The obtained solution washeated to 100° C., maintained at this temperature for 30 minutes, andallowed to cool down to 25° C., to thereby obtain a cerium suspension.

After the mother liquor was removed from the cerium suspension thusobtained, the total volume was adjusted to 1 L with deionized water;concentration of anions is hence decreased by 44%, in comparison withanions comprised in the liquid medium after heating.

Then the cerium suspension was maintained at 120° C. for 2 hours,allowed to cool, and neutralized to pH 8.5 with aqueous ammonia.

To a slurry resulting from the neutralization, 12.5 g of lauric acid wasadded, and stirred for 60 minutes.

The obtained slurry was subjected to solid-liquid separation through aNutsche filter to obtain a filter cake. The cake was calcined in the airat 300° C. for 10 hours to obtain cerium oxide powder.

The obtained composite oxide powder was measured of the specific surfacearea by the BET method after calcination at 800° C. for 2 hours and at900° C. for 5 hours.

Example 2

A cerium oxide powder was prepared in the same way as in Example 1except that the concentration of anions is decreased by 39%, incomparison with anions comprised in the liquid medium after heating.

The properties of the oxide powder thus obtained were evaluated in thesame way as in Example 1.

Example 3

A cerium oxide powder was prepared in the same way as in Example 1except that 12.5 g of capric acid instead of lauric acid was added.

The properties of the oxide powder thus obtained were evaluated in thesame way as in Example 1.

Comparative Example 1

A cerium oxide powder was prepared in accordance with the methoddisclosed in Patent Publication U.S. Pat. No. 7,361,322 B2.

20 g of a ceric nitrate solution in terms of CeO₂ containing not lessthan 90 mol % tetravalent cerium cations was measured out, and adjustedto a total amount of 1 L with deionized water. The obtained solution washeated to 100° C., maintained at this temperature for 24 hours, andallowed to cool down to the room temperature. Then aqueous ammonia wasadded to neutralize to pH 8 to obtain cerium oxide hydrate in the formof the slurry.

The slurry was then subjected to solid-liquid separation with a Nutschefilter to obtain a filter cake. The cake was calcined in the air at 300°C. for 10 hours to obtain cerium oxide powder.

The properties of the oxide powder thus obtained were evaluated in thesame way as in Example 1.

Comparative Example 2

A cerium oxide was prepared in the same way as in Comparative Example 1except that the 5.0 g of lauric acid was added after addition of aqueousammonia, and stirred for 60 minutes.

Comparative Example 3

A cerium oxide was prepared in the same way as in Comparative Example 1except that the mother liquor was removed after obtaining a ceriumsuspension.

Comparative Example 4

A cerium oxide was prepared in the same way as in Example 1 except thatthe mother liquor was not removed after obtaining a cerium suspension.

Properties of the cerium oxides prepared in the above defined examplesare mentioned in Table 1.

TABLE 1 Total pore Total pore SBET SBET TPR TPR volume volume 800° C./2h 900° C./5 h S1/S2 S1/S2 800° C./2 h 900° C./5 h (m²/g) (m²/g) 800°C./2 h 900° C./5 h (ml/g) (ml/g) Inv. 1 92 60 0.537 0.342 1.25 1.16 Inv.2 87 62 0.530 0.350 1.18 1.22 Inv. 3 101 57 0.544 0.343 1.31 1.19 Comp.1 75 46 0.488 0.259 0.73 0.72 Comp. 2 78 47 0.469 0.271 0.82 0.78 Comp.3 77 44 0.479 0.253 0.71 0.68 Comp. 4 73 48 0.463 0.268 0.79 0.77

Description of Analysis Method

BET:

The Specific surface area is measured by BET method in the followingway. Use is made of a Mountech Co., LTD. Macsorb analyzer with a 200 mgsample which has been calcined beforehand at 800° C. for 2 hours or 900°C. for 5 hours under air.

TPR:

The TPR is performed using a temperature programmed desorption analyzermanufactured by Okura Riken Co., LTD. with a carrier gas containing 90%argon and 10% hydrogen, at a gas flow rate of 30 ml/min, at a heatingrate of a sample during measurement of 13.3° C./min, and using 0.5 g ofa sample which has been calcined beforehand at 800° C. for 2 hours or900° C. for 5 hours under air.

S1/S2 ratio, that is, a ratio of the area (S1) defined by the baselineand the TPR curve in the temperature range of 200 to 600° C., to thearea (S2) defined by the baseline and the TPR curve in the temperaturerange of 600 to 1000° C.

The TRP curve is expressed in TCD (Thermal Conductivity Detector) signalfor Y axis and in temperature for X axis. A higher S1/S2 ratio of acerium oxide relates to a higher oxygen absorbing and desorbingcapability and higher activity to purify exhaust gas at a lowertemperature. As used herein, the “baseline” means a line segment drawnfrom the point on the TPR curve corresponding to 200° C. in a parallelto the axis representing temperature, up to 1000° C.

Hg Porosity:

The total pore volume is measured by mercury intrusion porosimetry inthe following way. Use is made of a Micromeritics AutoPore IV 9500 witha 200 mg sample which has been calcined beforehand at 800° C. for 2hours or 900° C. for 5 hours under air.

Example 2: Low Temperature NOx Storage Capacity Testing

Cerium oxides of example 1 and comparative example 1 are calcined underair at 800° C. for 4 h. The NOx storage capacity is then measured in thefollowing way: a synthetic gas mixture (30 L/h), representative of thecatalytic process with the composition A is flushed during 90 min at120° C. through 150 mg of cerium oxide placed in a fixed bed reactor.The amount of NOx stored is monitored on line in function of the time,owing to an Antaris IGS FTIR Spectrometer.

Composition A (vol %) NO 0.014 NO₂ 0.018 H₂O 5 CO₂ 5 O₂ 10 N₂ balance

NOx adsorption of both cerium oxides of example 1 and comparativeexample 1 at 90 mins is shown in Table 2:

TABLE 2 NOx adsorbed (μg NOx/g Ce oxide) Ex. 1 23.93 Comp. 1 21.52

It appears then that the cerium oxide of the present invention has ahigher NOx capture performance than conventional cerium oxide. NSC (NOxstorage capacity) is an indicator to evaluate the NOx emissionperformance.

The invention claimed is:
 1. A method for preparing cerium oxideparticles, the method comprising: heating a cerium salt solution at atemperature comprised between 60 and 220° C. to obtain an initialsuspension comprising a liquid medium and a precipitate, wherein thecerium salt solution comprises anions and cations and wherein between 90and 100 mol % of the cations are tetravalent cerium cations; decreasingthe concentration of anions in the initial suspension between 10 and 90mol %, thus forming a modified suspension; heating the modifiedsuspension at a temperature comprised between 100 and 300° C., to form aheated suspension; optionally cooling the heated suspension; bringingsaid heated suspension, optionally cooled, into contact with a basiccompound, thus forming a basic suspension; optionally separating a firstprecipitate from the liquid medium of the basic suspension; adding anorganic texturing agent to the basic suspension, thus forming a texturedsuspension, or to the first precipitate, thus forming a textured firstprecipitate; optionally separating a second precipitate from the liquidmedium of the textured suspension; and calcining the textured firstprecipitate or the second precipitate to obtain cerium oxide particles;wherein at least one of the optional separating steps is performed inthe method.
 2. The method according to claim 1 wherein the cerium saltsolution is selected from the group consisting of: cerium nitratesolution, cerium ammonium nitrate solution, cerium sulfate solution andcerium ammonium sulfate solution.
 3. The method according to claim 1wherein the cerium salt solution has a cerium concentration comprisedbetween 5 and 150 g/L in terms of cerium oxide.
 4. The method accordingto claim 1 wherein decreasing the concentration of anions in the initialsuspension comprises adding water to the initial suspension.
 5. Themethod according to claim 1 wherein decreasing the concentration ofanions in the initial suspension comprises removing at least a part ofthe liquid medium from the initial suspension and then adding water tothe medium.
 6. The method according to claim 1 wherein the organictexturing agent is selected from the group consisting of: anionicsurfactants, nonionic surfactants, polyethylene glycols, carboxylicacids and their salts, and carboxymethylated fatty alcohol ethoxylatesurfactants.
 7. Cerium oxide particles having the following properties:a specific surface area (SBET) comprised between 80 and 120 m²/g aftercalcination at 800° C. for 2 hours, under air; a specific surface area(SBET) comprised between 55 and 80 m²/g after calcination at 900° C. for5 hours, under air; a total pore volume comprised between 0.9 and 1.6ml/g after calcination at 800° C. for 2 hours, under air; and a totalpore volume comprised between 0.85 and 1.5 ml/g after calcination at900° C. for 5 hours, under air.
 8. Cerium oxide particles according toclaim 7 wherein said particles have an S1/S2 ratio comprised between 0.5and 0.7 taken after calcination at 800° C. for 2 hours, wherein S1 isthe area defined by a baseline and a TPR curve in a temperature range of200 to 600° C. and S2 is the area defined by said baseline and said TPRcurve in a temperature range of 600 to 1000° C., wherein the baseline isa line segment drawn from a point on the TPR curve corresponding to 200°C., parallel to the temperature axis, up to 1000° C., and wherein theTPR curve is the result of temperature programmed reduction of saidparticles using a temperature programmed desorption analyzer with acarrier gas containing 90% argon and 10% hydrogen at a gas flow rate of30 ml/min and at a heating rate of 13.3° C./min.
 9. Cerium oxideparticles according to claim 8 wherein said particles have an S1/S2ratio comprised between 0.3 and 0.5 taken after calcination at 900° C.for 5 hours.
 10. Cerium oxide particles according to claim 7 whereinsaid cerium oxide particles comprise at least one rare earth elementoxide, other than cerium oxide.
 11. A catalyst comprising at leastcerium oxide particles according to claim
 7. 12. The catalyst accordingto claim 11 wherein said catalyst is a co-catalyst comprising the ceriumoxide particles, for purifying exhaust gas.
 13. A composition comprisingat least cerium oxide particles according to claim
 7. 14. Compositionaccording to claim 13, wherein said composition is a suspensioncomprising at least a liquid medium and the cerium oxide particles. 15.A method of removing a portion of a substrate, comprising: providing atleast a composition comprising cerium oxide particles according to claim7, contacting at least the composition with the substrate to bepolished, and polishing the substrate.
 16. Cerium oxide particlesaccording to claim 7 wherein said particles have: a S1/S2 ratiocomprised between 0.5 and 0.7 taken after calcination at 800° C. for 2hours, and a S1/S2 ratio comprised between 0.3 and 0.5 taken aftercalcination at 900° C. for 5 hours, wherein S1 is the area defined by abaseline and a TPR curve in a temperature range of 200 to 600° C. and S2is the area defined by said baseline and said TPR curve in a temperaturerange of 600 to 1000° C., wherein the baseline is a line segment drawnfrom a point on the TPR curve corresponding to 200° C., parallel to thetemperature axis, up to 1000° C., and wherein the TPR curve is theresult of temperature programmed reduction of said particles using atemperature programmed desorption analyzer with a carrier gas containing90% argon and 10% hydrogen at a gas flow rate of 30 ml/min and at aheating rate of 13.3° C./min.
 17. Cerium oxide particles according toclaim 16 wherein said cerium oxide particles comprise at least one rareearth element oxide, other than cerium oxide.